Ignition control circuit for internal combustion engine

ABSTRACT

A combined heat and power system including a 4 cylinder reciprocating internal combustion engine fuelled by a fuel gas, for example natural gas, and operating at a substantially constant speed and at a substantially constant air to fuel ratio. Operation of a spark generator and distributor device (44) is under control of an ignition spark duration control (10) whereby the duration of an ignition spark between the electrodes of the plug (16) is automatically variable to bring the spark duration closer to a predetermined minimum duration over which predetermined minimum duration there would be supplied to the electrodes of the plugs in turn a minimum-level of electrical ignition energy to substantially avoid misfire in the cylinders provided with the plugs. The minimum level of the electrical ignition energy is variable according to the size of the gap between the electrodes of the plug (16). The duration of the ignition spark of the plug (16) is observed by a monitor (62) in the lead which inputs the observed duration time into the control (10).

This invention concerns a reciprocating internal combustion engine, andalso concerns combined heat and power (CHP) apparatus provided with suchan engine.

In such CHP apparatus the engine drives an electrical generator and alsoproduces utilizable heat.

An economic case for using CHP apparatus is considerably dependent onthe costs of routine maintenance of the apparatus, such as the cost ofservicing the engine. Extending the service intervals of the engine canhave a significant effect on the economic case for using CHP.

The spark plugs used in a CHP internal combustion engine need to bechecked regularly to ensure that the gap between the plug electrodesdoes not increase to a size where misfire occurs.

An object of the invention is to provide a reciprocating internalcombustion engine so adapted as to alter automatically the electricalignition energy applied to the electrodes of a spark plug in a cylinderof the engine to substantially a minimum level required to ensurecorrect combustion of the fuel in the cylinder.

According to the invention a reciprocating internal combustion engine,which operates at a substantially constant speed and at a substantiallyconstant air to fuel ratio, comprises at least one cylinder providedwith a spark plug having at least two electrodes with a gaptherebetween, and ignition spark duration control means whereby theduration of an ignition spark between said electrodes is automaticallyvariable to bring said spark duration closer to a predetermined minimumduration over which predetermined minimum duration there would besupplied to the electrodes a minimum level of electrical ignition energyto substantially avoid misfire in the cylinder, and said minimum levelbeing variable according to the size of said gap.

Thus there is avoided the need to apply excessive electrical ignitionenergy to the spark plug electrodes all the time in order to ensurethere is enough energy to cause an adequate spark when either or bothelectrodes is/are worn and the spark gap has increased over the manyhours of engine operation after the spark plug was fitted or itselectrode gap set. Accordingly the avoidance of the need to continuallyuse excessive electrical ignition energy slows the rate of spark plugelectrode wear and slows the rate of increase of electrode gap size, fora given spark plug.

Engine test work has shown that for a given engine operating underconstant conditions of speed, fuel to air ratio, (and also load) thereis a minimum, fixed spark duration necessary to ensure correctcombustion. For a given spark plug electrode gap, the fixed minimumspark duration requires a certain minimum electrical ignition energy tobe supplied. An increase in the supplied electrical energy beyond thisminimum level will not show any benefits, whereas a reduction will causemisfire. As the spark plug gap increases, then the said minimum energylevel required, increases.

The invention will now be further described, by way of example withreference to the accompanying drawings in which:

FIG. 1 is a diagrammatic representation of a combined heat and power(CHP) apparatus comprising a reciprocating, internal combustion engineformed according to the invention;

FIG. 2 diagrammatically shows ignition spark duration control means incombination with an ignition system and spark plugs of the engine inFIG. 1;

FIG. 3 is a diagrammatic representation of an ignition spark durationmonitor used in the apparatus in FIG. 2;

FIG. 4 is a set of graphs in which there is a common abscissa axis fortime t, a plurality of ordinate axes for the voltages v of a pluralityof different voltage signal outputs from respective component parts ofthe ignition spark duration control means in FIG. 2;

FIG. 5 is a diagrammatic representation of the arrangement in FIG. 2, asan aid to understanding FIGS. 5 and 6;

FIG. 6 is a diagrammatic representation of a fragment of a secondembodiment of an internal combustion engine formed according to theinvention, having an ignition system of the wasted spark type, and

FIG. 7 is a diagrammatic representation of a fragment of a thirdembodiment of an internal combustion engine formed according to theinvention in which each spark plug is supplied with electrical ignitionenergy from a respective ignition coil.

In the following description like references refer to like or similarparts.

With reference to FIG. 1 a combined heat and power (CHP) apparatus 2comprises a multi-cylinder, reciprocating internal combustion engine 4which may be fuelled by fuel gas, for example natural gas, and isarranged to operate at a substantially constant predetermined speed withsubstantially constant, predetermined air to fuel ratio, and may operateunder a substantially constant load. The engine 2 drives an electricalgenerator 6 and heat from the engine, for example from the enginecoolant and/or from the exhaust gases and/or from the lubricating oil,is extracted by a heat exchanger arrangement 8 for the heat to be usedfor some heating purpose.

The engine 2 is provided with an ignition spark duration control 10 tocontrol the duration of the ignition spark between two electrodes 12 and14 (see FIGS. 2 and 5) of a respective spark plug 16, 18, 20 or 22 ineach of the cylinders of the engine. Preferably the spark plugs aresubstantially similar in their operating characteristics.

With reference to FIG. 2, the electrode 12 in each spark plug isconnected by a respective plug lead 24, 26, 28 or 30 connected to arespective contact 32, 34, 36, 38 in a distributor 40 known per secomprising a rotor arm 42 in a spark generator 44 known per se whereinan high tension or king lead 46 to the rotor arm is from a secondarywinding 48 of an ignition coil 50 having a primary winding 52. Theignition coil 50 is part of an ignition coil powering system 54 (knownper se) comprising a battery or accumulator 56 of any suitable kind(known per se), often a lead-acid accumulator, an ignition switch 58(known per se) which is closed when the engine is running and atransistor switch 60 turned on and off by a signal of varying voltage online 62 from the ignition spark duration control 10.

The invention may be applied to engines of a standard type using a giventype of spark plugs so that one engine of that standard type may be usedas a typical example to establish an operating characteristic, namely apredetermined minimum ignition spark duration at the plugs. The engineused as the example is operated and the ignition spark duration is madeprogressively shorter until misfire occurs. Then the spark duration timeis increased until misfire just ceases to occur and the duration is thenmeasured. That measured duration is substantially the predeterminedminimum duration for the ignition spark the occurrence of which may besaid to ensure the correct combustion of the fuel.

With reference to FIGS. 2 and 3 an ignition spark duration monitor 62 isprovided in one of the plug leads, in this example in the lead 24. Thespark duration monitor 62 comprises a bridge rectifier 63 havingopposite arms connected to the plug lead 24, and the bridge-rectifierbeing also connected to a circuit network comprising an opticaltransmitter 64 in series with the spark plug 16. When electricalignition spark current flows in the lead 24 the optical transmitter 64performs its transducer function and produces an optical signaltransmitted along an optical fibre path 66 to the spark duration control10, the duration of the optical signal being substantially equal to theduration of the spark. The bridge rectifier 63 allows for sparks ofeither polarity, and a parallel resistor 68 and capacitor 70 across theoptical transmitter 64 are used to by-pass some of the spark current andhigh frequency spikes. The circuit network in the monitor 62 may alsoinclude a light emitting diode (LED) 72 to provide visual confirmationthat spark current is present. In an alternative arrangement, the sparkduration monitor may be mounted in the king lead 46 as exemplified byspark duration monitor 62¹ shown in dotted lines.

With reference to FIGS. 2 and 4, the optical signal on fibre optic path66 is input to a spark upto receiver 74 in which the optical signal isconverted to a corresponding electrical voltage signal output Arepresenting the spark current and illustrated in FIG. 4A which istypically a block or square wave 76 followed by a burst of (current)spikes 78. The spikes form part of the total spark duration and occurbecause the optical transmitter 64 (FIG. 3) has a high threshold toavoid interference, and does not detect low current levels in plug lead24 when the spark energy is decaying. The output signal A is input tospark duration conditioning circuit 80 giving a voltage output squarewave signal B illustrated at 82 in FIG. 4B and lasting for substantiallythe duration of the spark (ie. a simulated reconstitution of the spark).

A constant current generator 84 inputs to a spark duration ramp circuit86 which produces at C a voltage ramp output signal, represented at 88in FIG. 4C, which cuts off at the cut-off of the square wave 82. A peakvalue, at 90, of the ramp signal 88 is detected by a peak detectorcircuit 92. The peak value 90 is a function of or proportional to thespark duration, and the peak detector circuit 92 gives an output D at aD.C. voltage level, represented by square wave 94 in FIG. 4D, which maybe the same as the voltage level 90 but is in any case a function of orproportional to the spark duration. The signal 94 continues at the sameD.C. level until it cuts off at the instant of commencement of theobservation of the next spark duration signal 76¹, 78¹. The level of thesignal 94 may be stored in the peak detector circuit 92 in a sample andhold integrated circuit chip using a capacitor to store the voltagelevel. The signal 94 is input to a spark duration error generatorcircuit 96 to which is input at E (FIG. 2) an output signal from atarget spark duration signal generator 98 giving a signal at E (FIG. 2)(shown at 100 in FIG. D) which represents the predetermined, desiredminimum spark duration. In the circuit 96 the signal 94 is compared withthe minimum spark duration signal 100 to produce an error signal voltage102 which is added to a predetermined constant offset voltage value 104produced by an offset signal generator 106. The resultant voltage isinput to a time constant circuit 108, for example a ten second timeconstant circuit, to give an output signal at H represented in FIG. 4Hby a dwell modifier voltage signal 110. "Dwell" in this context meansthe time duration over which the primary winding 52 of the ignition coilis energised. Another input to the time constant circuit 108 is providedthrough a two-position switch 101 which in the position shown in FIG. 2allows an input to the circuit 108 which allows the circuit to give theoutput signal 110 (FIG. 4H) at H; but in the other position the switch101 causes disablement of the time constant circuit 108 which thus doesnot give an output at H and thus places the system 74, 80, 84, 86, 92,96, 108 in disabled mode.

The time constant of the time constant circuit 108 produces an averageerror signal allowing a slow correction of spark duration with improvedstability.

Continuing with reference to FIG. 2, the ignition spark duration control10 includes an ignition dwell conditioning circuit 112 which may beoriginal equipment (OE) provided by the engine manufacturer. Theignition dwell conditioning circuit 112 is triggered to give itselectrical voltage signal output in response to signals from an enginecrank shaft angle monitor 114. The monitor 114 observes rotation of thecrank shaft and gives an output signal when the crank shaft is at eachone in turn of a number of predetermined angles of rotation at each ofwhich the spark generating procedure for the respective spark plug 16,18, 20 or 22 should be commenced. That output from monitor 114 is inputto the ignition dwell conditioning circuit 112 to trigger it to give avoltage output signal at F represented in FIG. 4F by the square waveignition signal 116 of a constant, predetermined duration. A constantcurrent generator 118 inputs to an OE voltage ramp generator 120 whichproduces an output G shown at 122 in FIG. 4G as a ramp voltage ofconstant slope, for the duration of the square wave 116. The rampvoltage signal 122 is input to a dwell comparator circuit 124 to whichthe dwell modifier signal 110 is also input. When the voltage ramp 122reaches a voltage value equal to that of the dwell modifier signal 110the dwell comparator gives a voltage output J represented in FIG. 4J bya square wave modified ignition signal 124 which goes low when theignition signal 116 goes low. The modified ignition signal 124 isamplified by ignition amplifier 126 and the output of the amplifier 126is input on line 62 to the base of the transistor switch 60 whichswitches on for the duration of the modified ignition signal 124 so thatcurrent flows in the primary winding 52 of the ignition coil 50 for thatduration which determines the amount of magnetic energy stored in theprimary winding. That amount of magnetic energy is enough so that whenit decays when the transistor switch 60 is turned off at the cut-off ofthe modified ignition signal 124 (FIG. 4J) the voltage induced in thesecondary winding 48 is enough to produce an ignition spark, at one ofthe spark plugs 16, 18, 20 or 22, of a duration which is substantiallythe predetermined minimum or is approaching said predetermined minimum.

In FIGS. 2 and 5 only the spark duration of the plug 16 is observed bythe monitor 62, so the ignition spark duration of the plug 16 determinesthe magnetic energy stored in the primary winding 52 in the course ofthe firing procedures of the other three plugs 18, 20 and 22.

When the ignition spark duration varies from the said predeterminedminimum duration represented by the voltage value of the minimum sparkduration signal 100 (FIG. 4D) output from the target spark durationsignal generator 98, it causes the error voltage 102 to vary whichresults in a variation of the duration of the modified ignition signal124. In turn this causes variation in the amount of magnetic energystored on the primary winding 52 to an amount which will result in theignition spark, created as a consequence of the decay of that storedmagnetic energy, having a duration which more closely approaches thepredetermined minimum duration.

The offset voltage signal 104 has a constant voltage value whichprovides a safety factor by substantially ensuring that the dwellmodifier voltage signal 110 (FIG. 4H) does not fall below to a valuewhich can result in misfire.

In FIG. 6 the spark plugs 16, 18, 20 and 22 are fired in accordance withthe wasted spark method. The crank angle monitor 114 inputs signals intoan electronic selection means 128 which gives two outputs each fed intoa respective spark duration and generating system 130A or 130B. Thesystem 130A comprises an ignition spark duration control 10 as describedabove connected to an ignition coil powering system 54 in which there isno distributor 40 (FIG. 2) and instead opposite ends of the secondarywinding 48 are respectively connected to the plug leads 24 and 26; andthe monitor 62 is in the lead 24. When the secondary coil 48 in thesystem 134A is energised both plugs 16 and 18 fire simultaneously.

Likewise the system 130B comprises an ignition spark control 10 aspreviously described connected to an ignition powering system 54 as inthe system 130A but this time opposite ends of the secondary winding arerespectively connected to the plug leads 28 and 30 which latter includesthe monitor 62. In each system 130A and 130B the ignition spark durationcontrol 10 therein operates in like manner to the control in FIG. 2described above. But in this case the system 130A controls the durationof the ignition sparks at the spark plugs 16 and 18 and the system 130Bcontrols the duration of the sparks at the plugs 20 and 22. The crankshaft angle monitor 114 observes rotation of the crank shaft through thevarious angular positions (known per se) at each of which the sparkgeneration procedure for either one or the other set of plugs 16, 18 or20, 22 should be commenced and the selection means 128 in responseautomatically selects to which system 130A or 130B it sends an outputsignal to trigger the ignition dwell conditioning circuit 112 in theselected system 130A or 130B. Although two ignition switches 58 andbatteries 56 are shown, a single ignition switch and battery arepreferably common to both systems 130A and 130B. Also it will beunderstood that the minimum electrical ignition energy supplied byeither secondary winding 48 to fire either pair of spark plugs 16, 18 or20, 22 is that required to fire two spark plugs.

In FIG. 7 each spark plug 16, 18, 20 or 22 is connected to a respectivespark duration and generating system 130C, 130D, 130E or 130F so thatthe secondary winding 42 in any of those systems can only fire oneparticular spark plug. The crank angle monitor observes rotation of thecrank shaft through various angular positions (known per se) at each ofwhich the spark generating procedure for a respective spark plug 16, 18,20 or 22 should be commenced and the electronic selection means 128 inresponse automatically selects which system 130C, 130D, 130E or 130F itsends an output signal to to trigger the ignition dwell conditioningcircuit 112 in the selected system 130c, 130D, 130E or 130F. Althoughfour ignition switches 58 and four batteries 56 are shown, a singleignition switch and a single battery are common to the four systems130C, 130D, 130E and 130F.

I claim:
 1. A reciprocating internal combustion engine comprising:atleast one cylinder provided with a spark plug having at least twoelectrodes with a gap therebetween; ignition spark duration controlmeans whereby the duration of an ignition spark between said electrodesis automatically variable to bring said spark duration closer to apredetermined minimum duration during which predetermined minimumduration there is supplied to the electrodes a minimum level ofelectrical ignition energy to substantially avoid misfire in thecylinder, and wherein said minimum level is variable according to thesize of said gap; and duration observing means comprising optical signalgenerating means, said duration observing means determining the durationof a phenomenon which is a function of said predetermined duration ofsaid spark and producing an optical signal which is a durationrepresentative signal which is a function of said duration of saidphenomena, and wherein said ignition spark duration control meansreceives said duration representative signal.
 2. An internal combustionengine according to claim 1, arranged to operate at a substantiallyconstant load.
 3. An internal combustion engine according to claim 1,wherein said duration observing means determines the duration of sparkproducing electrical energy discharged from a secondary winding of anignition coil.
 4. An internal combustion engine according to claim 3, inwhich said duration observing means determine the duration of sparkproducing electrical energy in an electrical plug lead connected to saidspark plug.
 5. An internal combustion engine according to claim 1,wherein there is a plurality of said cylinders each with a respectivesaid spark plug.
 6. An internal combustion engine according to claim 5,wherein each said spark plug is connected to a respective said pluglead, said duration observing means is arranged to observe the durationof spark producing energy in one of said plug leads, and said ignitionspark duration control means being common to the said spark plugswhereby a given amount of electrical energy constituting said minimumlevel is supplied to each of said spark plugs in turn.
 7. An internalcombustion engine according to claim 4, in which there are two saidcylinders each provided with a respective said spark plug, each plug isconnected to a respective said plug lead, both plug leads are connectedto the secondary winding which is arranged to fire the two spark plugssimultaneously, and said ignition spark duration control means isarranged to control the duration of the ignition spark at each of thesaid spark plugs and utilize the duration representative signal producedby the duration observing means observing the duration of the electricalspark producing energy supplied to the spark plugs.
 8. An internalcombustion engine according to claim 4, in which there is a plurality ofsaid cylinders each with a respective said spark plug each connected bya respective said plug lead to a said secondary winding of a respectivesaid ignition coil, each ignition coil is arranged for operation inresponse to signals provided by a respective said ignition sparkduration control means, each ignition spark duration control means isconnected to a respective said duration observing means, and eachduration observing means is arranged to observe the duration of sparkproducing electrical energy in a respective said plug lead.
 9. Aninternal combustion engine according to claim 3, in which the durationrepresentative signal is utilized in said ignition spark durationcontrol means to produce an error signal when the durationrepresentative signal differs from said predetermined minimum duration,and a control signal which is a function of said error signal iscompared with a dwell signal which would initiate provision of asubstantially fixed supply of said electrical ignition energy, andshould said error signal vary from said dwell signal then an ignitioncontrol signal is produced to cause the electrical ignition energy to bevaried to a value which results in the duration of the ignition sparkapproaching nearer to or being substantially equal to said predeterminedminimum duration.
 10. An internal combustion engine according to claim1, in which the optical signal generating means is included in a hightension lead from the ignition coil.
 11. An internal combustion engineaccording to claim 1, in which said duration observing means determinesthe duration of spark producing electrical energy in an electrical pluglead connected to said spark plug.
 12. An internal combustion engineaccording to claim 2, in which said duration observing means determinesthe duration of spark producing electrical energy in an electrical pluglead connected to said spark plug.
 13. An internal combustion engineaccording to claim 4, in which the duration representative signal isutilized in said ignition spark duration control means to produce anerror signal when the duration representative signal differs from saidpredetermined minimum duration, and a control signal which is a functionof said error signal is compared with a dwell signal which wouldinitiate provision of a substantially fixed supply of said electricalignition energy, and should said error signal vary from said dwellsignal then an ignition control signal is produced to cause theelectrical ignition energy to be varied to a value which results in theduration of the ignition spark approaching nearer to or beingsubstantially equal to said predetermined minimum duration.
 14. Aninternal combustion engine according to claim 5, in which the durationrepresentative signal is utilized in said ignition spark durationcontrol means to produce an error signal when the durationrepresentative signal differs from said predetermined minimum duration,and a control signal which is a function of said error signal iscompared with a dwell signal which would initiate provision of asubstantially fixed supply of said electrical ignition energy, andshould said error signal vary from said dwell signal then an ignitioncontrol signal is produced to cause the electrical ignition energy to bevaried to a value which results in the duration of the ignition sparkapproaching nearer to or being substantially equal to said predeterminedminimum duration.
 15. An internal combustion engine according to claim6, in which the duration representative signal is utilized in saidignition spark duration control means to produce an error signal whenthe duration representative signal differs from said predeterminedminimum duration, and a control signal which is a function of said errorsignal is compared with a dwell signal which would initiate provision ofa substantially fixed supply of said electrical ignition energy, andshould said error signal vary from said dwell signal then an ignitioncontrol signal is produced to cause the electrical ignition energy to bevaried to a value which results in the duration of the ignition sparkapproaching nearer to or being substantially equal to said predeterminedminimum duration.
 16. An internal combustion engine according to claim7, in which the duration representative signal is utilized in saidignition spark duration control means to produce an error signal whenthe duration representative signal differs from said predeterminedminimum duration, and a control signal which is a function of said errorsignal is compared with a dwell signal which would initiate provision ofa substantially fixed supply of said electrical ignition energy, andshould said error signal vary from said dwell signal then an ignitioncontrol signal is produced to cause the electrical ignition energy to bevaried to a value which results in the duration of the ignition sparkapproaching nearer to or being substantially equal to said predeterminedminimum duration.
 17. An internal combustion engine according to claim8, in which the duration representative signal is utilized in saidignition spark duration control means to produce an error signal whenthe duration representative signal differs from said predeterminedminimum duration, and a control signal which is a function of said errorsignal is compared with a dwell signal which would initiate provision ofa substantially fixed supply of said electrical ignition energy, andshould said error signal vary from said dwell signal then an ignitioncontrol signal is produced to cause the electrical ignition energy to bevaried to a value which results in the duration of the ignition sparkapproaching nearer to or being substantially equal to said predeterminedminimum duration.
 18. Combined heat and power apparatus comprising areciprocating internal combustion engine comprising:at least onecylinder provided with a spark plug having at least two electrodes witha gap therebetween; ignition spark duration control means whereby theduration of an ignition spark between said electrodes is automaticallyvariable to bring said spark duration closer to a predetermined minimumduration during which predetermined minimum duration there is suppliedto the electrodes a minimum level of electrical ignition energy tosubstantially avoid misfire in the cylinder, and wherein said minimumlevel is variable according to the size of said gap; and durationobserving means comprising optical signal generating means, saidduration observing means determining the duration of a phenomenon whichis a function of said predetermined duration of said spark and producingan optical signal which is a duration representative signal which is afunction of said duration of said phenomena, and wherein said ignitionspark duration control means receives said duration representativesignal.